There is a considerable amount of under-utilized thermal energy from high-temperature heat sources (>700° C.), e.g., nuclear power plants, because the maximum operating temperature for steam turbines is typically below 650° C. (primarily limited by the corrosiveness of high pressure, high temperature steam).
Thermochemical cycles for water splitting or CO2 reduction are able to convert thermal energy into chemical energy stored in hydrogen and CO, respectively. Water (CO2) is split into stoichiometric amounts of hydrogen (CO) and oxygen in a series of chemical reactions via a closed thermochemical cycle, with heat as the only energy input. No other products are produced in these cycles.
Two types of thermochemical cycles are generally used for this purpose: high-temperature, two-step cycles and low-temperature, multistep cycles. The former usually employs relatively simple reactions and benign chemicals, e.g., transition metals and metal oxides; however, the operating temperature required to close the cycle is typically higher than 1500° C. Currently, heat sources with such high temperatures, e.g., high temperature solar concentrators, are still scarce. In contrast, heat sources at temperature range of 700-1000° C. are much more abundant, e.g., nuclear power plants and medium-scale solar concentrators. In addition, high-temperature operating fluids, e.g., molten salts, have been developed to work in this temperature range. Low-temperature multistep thermochemical cycles are designed to operate at 700-1000° C.; however, the toxic and corrosive chemicals involved pose significant environmental and engineering challenges. For example, in each reaction of the three-step sulfur-iodine thermochemical cycle for water splitting, with a highest operating temperature of 850° C., at least one of the following chemicals are involved: H2SO4, HI, SO2 and I2.
There is a need for thermochemical water splitting cycles that involve non-corrosive solids and operate at below 1,000° C. The disclosed inventions address these needs.